Unable to run ffmpeg with qProcess to extract a single frame from Video

kulvi

I am trying to extract a single frame from a video using the Qprocess and ffmpeg and store it in a tmp folder. When I run the program in terminal command line it is able to extract it but not when qprocess runs it. I'm developing on a mac. What am I doing wrong.

QString extractSingleFrame(QString videoPath, QDir tmpDir)
{
    QString program = ffmpegPath();
    QProcess *ffmpegProcess = new QProcess();
    QString arguments;
    QFileInfo videoInfo(videoPath);
    QString videoName = videoInfo.fileName();

    QString firstFrameName(tmpDir.absolutePath() + "/" + videoName + ".jpg");
    arguments = " -y -i " + QDir::toNativeSeparators(videoPath)
                + " -frames:v 1 -q:v 1 -f image2 " + QDir::toNativeSeparators(firstFrameName);

    QFile::remove(firstFrameName);
    qDebug() << "Starting program" << program + arguments;
    ffmpegProcess->start(program + arguments);

    if (ffmpegProcess->state() == QProcess::Starting){
       qDebug() << "program is starting" + ffmpegProcess->state();
    }
    ffmpegProcess->waitForFinished(-1);

    qDebug() << "done";

    delete ffmpegProcess;

    return firstFrameName;
}

This is the printed command
Starting program "ffmpeg -y -i "/Users/userX/test.mov" -vframes 1 -q:v 1 -f image2 "/var/folders/s1/vtv2cx36p3h0000gn/T/test-ywDBgj/test.mov.jpg""

mrjj

Hi
You seems to append all parameters to one string.
It might be the reason.
Normally its a QStringList and given as a separate parameter.

 QObject *parent;
    ...
    QString program = "./path/to/Qt/examples/widgets/analogclock";
    QStringList arguments;
    arguments << "-style" << "fusion";

    QProcess *myProcess = new QProcess(parent);
    myProcess->start(program, arguments);

kulvi

I have tried to do:

ffmpegProcess->start(program , QStringList()
                         << "-y"
                         << "-i"
                         << QDir::toNativeSeparators(videoPath)
                         << "-frames:v 1"
                         << "-q:v 1"
                         << "-f"
                         << "image2"
                         << QDir::toNativeSeparators(firstFrameName));

This just gives the same result. In both cases ffmpegProcess->state() == QProcess::NotRunning gives a true flag.

I'm having a hard time getting some error message output to pinpoint this issue. I am new to Qt so further help would be deeply appreciated.
Using this to debug this gives me only an empty string:

QString StdOut_sdcompare = ffmpegProcess->readAllStandardOutput();
    qDebug() << StdOut_sdcompare;
    QString StdError_sdcompare = ffmpegProcess->readAllStandardError();
    qDebug() << StdError_sdcompare;

SGaist

Hi,

You still don't generate the parameter list correctly:

    QProcess ffmpeg;
    ffmpeg.start("ffmpeg" , QStringList()
                         << "-y"
                         << "-i" << QDir::toNativeSeparators(videoPath)
                         << "-frames:v" << "1"
                         << "-q:v" << "1"
                         << "-f" << "image2"
                         << QDir::toNativeSeparators(firstFrameName));
    ffmpeg.waitForFinished(-1);

kulvi

Thanks that did the trick, also I wasn't passing in the full path of ffmpeg.